1. Field of the Invention
The present invention is concerned with modulation schemes for the transmission of digital data, more specifically those using quadrature amplitude modulation (QAM) in which the transmitted signal consists of a series of discrete signal portions ("symbols") in each of which a carrier assumes a selected phase and amplitude. More specifically, the phase and amplitude of a symbol are constrained to be equal to a selected one of a set of phase/amplitude combinations. This set is usually referred to as a "constellation" and may be depicted graphically as a set of points on a plot in which cartesian coordinates represent in-phase and quadrature components (or polar coordinates represent phase and amplitude).
2. Related Art
In the simplest of such systems, the number of points in the (single) constellation is a power of two (e.g. 2.sup.n) and each group of n bits to be transmitted selects the point for each symbol (where each symbol thus carries n bits). In more sophisticated systems additional bits may be generated by redundant-coding process; as a result (or otherwise) the number of bits to be carried per symbol may be non-integer, and bits to be carried by a number of symbols need to be processed together. Such an arrangement results in a framing structure for the transmitted signal and necessitates provision for frame synchronisation in order that the symbols can be correctly interpreted at a receiver. Alternatively (or in addition) the data to be sent may have an inherent framing structure so that synchronisation is required for the data to be correctly processed at a receiver.